well, what does the Creative Zen Vision M read, what formats? .avi .mpeg .mov Its easier to find a free one if you know that then to buy one that can convert all sorts of formats when all u need is one. (not guranteeing that you'll find a free one either tho)
EDIT: okay I got unlazee and did a simple google search and got this info "MPEG-4, WMV, Motion JPEG, DivX, and Xvid" and I suppose its only nice to link to where I got that info <
http://www.engadget.com/2005/12/07/creatives-new-zen-vision-m-with-video/ > but I also read somewhere that Vision M has its own converter (check CD maybe) so you should already have most of everything you need.
however, if you are referring to Creatives converter not being all that great, then I would have to agree Mithrandir and ask what it is you call a "Good Video Converter" :\